Chapter 25 Alkanes and Cracking 25.2 The petrochemical industry 25.3 Cracking in petrochemical industry 25.4 Cracking in laboratory 25.5 Household gaseous fuels CONTENTS OF CHAPTER 25

25.1 ALKANES INTRODUCING ALKANES Petroleum and natural gas contain lots of hydrocarbons, most of which are alkanes. ALKANES are hydrocarbons with the general formula CnH2n+2. Figure 25.1 Space-filling models of methane, ethane, propane and butane. 25.1 ALKANES

PHYSICAL PROPERTIES OF ALKANES Table 25.1 Physical properties of some straight-chain alkanes. 25.1 ALKANES

Melting point, boiling point and density Figure 25.2 Some useful alkanes (or mixtures of alkanes): (a) Butane gas (liquefied) in a gas lighter (b) Baby oil (c) Candle wax. 25.1 ALKANES

CHEMICAL PROPERTIES OF ALKANES Viscosity The viscosity of liquid alkanes increases with a greater number of carbon atoms. Solubility Alkanes are insoluble in water. On the other hand, alkanes are soluble in many non-aqueous solvents (e.g. methylbenzene). CHEMICAL PROPERTIES OF ALKANES Alkanes are saturated hydrocarbons. They are quite unreactive. 25.1 ALKANES

Figure 25.3 Sodium reacts with air but not with alkanes. That is why sodium is stored under paraffin oil (a mixture of liquid alkanes). 25.1 ALKANES

CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) Combustion Alkanes are often used as fuels. The general equation for the complete combustion of alkanes (or other hydrocarbons) is: CxHy + (x + ) O2  xCO2 + H2O For example, CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) y 4 y 2 25.1 ALKANES

Figure 25.4 (a) LPG (consisting of lower alkanes) burns with a non-sooty blue flame. (b) A candle (consisting of higher alkanes) burns with a sooty yellow/orange flame. 25.1 ALKANES

Reaction with halogens 2C4H10(g) + 13O2(g)  8CO2(g) + 10H2O(l) Decomposition by heat On strong heating, higher alkanes decompose into simpler hydrocarbons. Reaction with halogens In diffuse sunlight, hexane reacts with bromine solution (in 1,1,1-trichloroethane). This is indicated by the disappearance of the red-orange colour of bromine. 25.1 ALKANES

hexane + bromine in 1,1,1-trichloroethane sunlight hexane + bromine in 1,1,1-trichloroethane red-orange colour of bromine is discharged red-orange colour of bromine is not discharged in darkness (a) (b) Figure 25.5 Hexane reacts with bromine in sunlight but not in the dark. 25.1 ALKANES

25.1 ALKANES

CH3Br + Br2  CH2Br2 + HBr dibromomethane CH2Br2 + Br2  CHBr3 + HBr tribromomethane CHBr3 + Br2  CBr4 + HBr tetrabromomethane The above reactions are examples of a type of reaction called substitution reaction. A SUBSTITUTION REACTION is a chemical change in which an atom (or a group of atoms) of an organic molecule is replaced by another atom (or group of atoms). 25.1 ALKANES

Substitution reactions are typical reactions of alkanes. HALOGENATION of an alkane is the substitution of a hydrogen atom in the alkane molecule by a halogen atom. A25.2 Chloromethane, dichloromethane, trichloromethane, tetrachloromethane and hydrogen chloride. 25.1 ALKANES

Chemical properties of hexane. Other alkanes behave similarly.

CHARACTERISTICS OF A HOMOLOGOUS SERIES — A BRIEF SUMMARY (1) All members of a series can be represented by the same general formula. (2) Each member differs from the next by a – CH2 – group. (3) Members show a gradual change of physical properties with increasing relative molecular mass. (4) All members have similar chemical properties, though higher members are less reactive. (5) All members can usually be obtained by the same general methods. 25.1 ALKANES

25.2 THE PETROCHEMICAL INDUSTRY Today, petroleum is the main source of a wide range of chemicals (petrochemicals). The industry that separates petroleum into fractions and changes them into other compounds is called the petrochemical industry. 25.2 THE PETROCHEMICAL INDUSTRY

Figure 25.6 Petrochemical industry consists of many plants, occupying a large area. 25.2 THE PETROCHEMICAL INDUSTRY

Petrochemical industry is divided into two main parts: (1) Oil refining This separates crude oil into fractions by fractional distillation. (2) Conversion processes Some of the oil fractions undergo further processes to make many useful products.  Cracking Heavy fractions (those with high boiling point ranges) may be cracked. CRACKING is the process of breaking down large molecules (usually long-chain organic molecules) into smaller ones. 25.2 THE PETROCHEMICAL INDUSTRY

Hong Kong town gas is currently made by this method.  Gasification of oil fraction In this process, an oil fraction (e.g. naphtha) is changed into a gaseous fuel. Hong Kong town gas is currently made by this method.  Conversion of alkenes Alkenes, formed in cracking processes, are useful starting materials for making a great variety of organic chemicals. 25.2 THE PETROCHEMICAL INDUSTRY

Figure 25.7 A variety of petroleum products. 25.2 THE PETROCHEMICAL INDUSTRY

25.3 CRACKING IN PETROCHEMICAL INDUSTRY SUPPLY AND DEMAND OF OIL FRACTIONS 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

Figure 25.8 Comparison of supply and demand of different oil fractions. 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

Heavy oils (in less demand) can be cracked to provide more petrol or kerosene (in greater demand). CRACKING OF OIL Cracking refers to the process of heating organic compounds in the absence of air. Usually a catalyst of aluminium oxide mixed with silicon(IV) oxide is also added. (The process is thus called catalytic cracking.) 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

Figure 25.9 Catalysts can speed up reactions. 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

small molecules out heated catalyst large molecules in Figure 25.10 A catalytic cracker at an oil refinery. 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

Manganese(IV) oxide can catalyse the decomposition of hydrogen peroxide into oxygen and water. (Oxygen can relight a glowing splint.) 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

Heavy fractions such as fuel oil are usually cracked to produce petrol Heavy fractions such as fuel oil are usually cracked to produce petrol. Use CH3(CH2)8CH3 as an example. 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

Figure 25.11 A decane molecule may be cracked at various points along the chain. Here are two of the many possible ways. 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

Importance of cracking More and more petrol is required as fuel for the ever-increasing number of motor vehicles. Importance of cracking Cracking is very important in the petroleum industry for two reasons:  To produce extra petrol Heavy fractions in less demand can be cracked to produce extra petrol.  To produce alkenes Cracking always produces alkenes. Alkenes (especially ethene and propene) can be used to make many useful organic chemicals. 25.3 CRACKING IN PETROCHEMICAL INDUSTRY

25.4 CRACKING IN LABORATORY Liquid paraffin is a mixture of alkanes. Molecules in the paraffin vapour break down on a hot porcelain surface. The products are lower alkanes and alkenes. 25.4 CRACKING IN LABORATORY

broken pieces of unglazed porcelain rocksil soaked with liquid paraffin strong heat gaseous products obtained from cracking water Figure 25.13 Cracking liquid paraffin in the laboratory. 25.4 CRACKING IN LABORATORY

A25.4 (a) An oil fraction. (b) The first few cm3 of gas is mainly air expelled from inside the apparatus. (c) The delivery tube should be removed from water before stopping to heat. This is to prevent sucking back of water which may crack the hot reaction tube. 25.4 CRACKING IN LABORATORY

Cracking medicinal paraffin and testing for flammability of product. 25.4 CRACKING IN LABORATORY

One purpose of cracking heavy oil fractions is to break alkane molecules by heat into smaller molecules, which are more flammable. 25.4 CRACKING IN LABORATORY

25.5 HOUSEHOLD GASEOUS FUELS LPG AND TOWN GAS In Hong Kong, the commonest domestic fuels are LPG (Liquefied Petroleum Gas) and town gas. LPG is a mixture of mainly propane and butane liquefied under pressure. 25.5 HOUSEHOLD GASEOUS FUELS

Figure 25.15 A LPG tank. 25.5 HOUSEHOLD GASEOUS FUELS

HONG KONG TOWN GAS Figure 25.16 The town gas plant at Tai Po Industrial Estate in Hong Kong. 25.5 HOUSEHOLD GASEOUS FUELS

How is town gas made in Hong Kong?  The raw material Naphtha (used for Hong Kong town gas production) is a mixture of C5 to C10 alkanes, mainly pentane and hexane.  Steam reforming of town gas Liquid naphtha is heated. The vapour produced is mixed with steam and passed over a hot nickel catalyst bed (at 700oC). Hydrogen, methane, carbon dioxide and carbon monoxide are formed. The following are typical reactions: C5H12(g) + 5H2O(g)  5CO(g) + 11H2(g) 2CO(g) + 2H2(g)  CO2(g) + CH4(g) 25.5 HOUSEHOLD GASEOUS FUELS

 Composition of town gas Table 25.2 Typical composition of Hong Kong town gas. 25.5 HOUSEHOLD GASEOUS FUELS

(a) LPG: a fraction from refining of petroleum; HK town gas: steam reforming of naphtha. (b) LPG: mainly propane and butane; Hong Kong town gas: mainly hydrogen and methane. 25.5 HOUSEHOLD GASEOUS FUELS

SUMMARY 1. On ascending the alkane series from methane to higher members, there is an increase in melting point, boiling point, density and viscosity (for liquid members). 2. Alkanes are quite unreactive. However, they can react with halogens (in sunlight) and burn in air. 3. A substitution reaction is a chemical change in which an atom (or a group of atoms) of an organic molecule is replaced by another atom (or group of atoms). Example: CH4 + Br2  CH3Br + HBr SUMMARY

4. All alkanes have similar chemical properties, but alkanes with larger molecules react more slowly. 5. There are certain characteristics of a homologous series. Please refer to p. 51. 6. There is a greater demand than supply for the following oil fractions:  Petrol  Kerosene  Gas oil 7. Cracking is the process of breaking down large molecules (usually long-chain organic molecules) into smaller ones. SUMMARY

 It produces extra petrol (as motor vehicle fuel) 8. Cracking heavy oils produces lighter oil fractions. That is, cracking long-chain alkanes produces short-chain alkanes. Alkenes are also formed. 9. Cracking is important in the petrochemical industry for two reasons:  It produces extra petrol (as motor vehicle fuel)  It produces alkenes (as starting materials to make a great variety of organic chemicals) 10. We can crack liquid paraffin using a simple laboratory set-up. Liquid paraffin requires a wick to burn. After cracking, the gaseous products are flammable. SUMMARY

C5H12(g) + 5H2O(g)  5CO(g) + 11H2(g); 11. Hong Kong town gas is made by the steam reforming of naphtha. Two typical reactions in the process: C5H12(g) + 5H2O(g)  5CO(g) + 11H2(g); 2CO(g) + 2H2(g)  CO2(g) + CH4(g) 12. Hong Kong town gas contains mainly hydrogen (~49%) and methane (~29%). It is poisonous because it contains carbon monoxide (3%) as well. SUMMARY